In the case where a circular machining or circular-arc machining operation is carried out by an end mill in a numerically controlled machine tool, slight backlashes of a mechanical system, delays in a follow-up operation, errors by the characteristics of a servo motor, or the like arise every time a workpiece is shifted a quadrant as defined between 0 degree, 90 degrees, 180 degrees and 270 degrees. With the result that, there becomes a problem that a quadrantal protuberance such as a protuberance and undercut occurs on the machined surface.
To correct such a quadrantal protuberance, a conventional NC apparatus has a function of correcting a quadrantal protuberance. For the correction for a quadrantal protuberance, a plurality of feeding operations are corrected on the basis of the amount of quadrantal protuberance correction, whereby the quadrantal protuberance is corrected.
However, the amount of a protuberance, i.e., the size of a quadrantal protuberance varies with machine tools. For this reason, the amount of a quadrantal protuberance correction is input to an NC apparatus in the form of a parameter according to the machine tools.
Where a perfectly-circular machining operation is carried out at high speed, for example, where a perfectly-circular machining operation is carried out at a feed rate of 10 m/min., a diameter of a resultantly finished workpiece becomes smaller than a programmed diameter, which results in a low degree of accuracy. This is attributable to the fact that errors arise in a feed mechanism according to a programmed geometry and feed rate.
To correct these errors, a conventional NC apparatus is so arranged as to be able to utilize a geometrical error correction function.
In general, this geometrical error correction function is optionally available and can be used simultaneously when correcting a quadrantal protuberance.
A quadrantal protuberance arises unless a quadrantal protuberance correction is carried out regardless of using the geometrical error correction function. However, the amount of a quadrantal protuberance differs according to whether or not the geometrical error correction function is used. Therefore, it is desirable to control the amount of quadrantal protuberance correction according to the amount of a quadrantal protuberance in each case.
However, a conventional NC apparatus is capable of storing only one set of the amount of a quadrantal protuberance, and hence it is not easy to control the amount of quadrantal protuberance correction according to the amount of the quadrantal protuberances, which in turn makes it impossible to automate the adjustment of the extent of quadrantal protuberance correction. For these reasons, the efficiency of machining operations is apt to become reduced.
In the current state of the art, the mean value between the amount of quadrantal protuberance correction where the geometrical error correction function is used and the amount of quadrantal protuberance correction where the geometrical error correction function is not used is input instead of adjusting the amount of quadrantal protuberance correction. Because of this, it is impossible to appropriately correct a quadrantal protuberance depending on the presence of geometrical errors to be corrected, which in turn makes it impossible to improve the accuracy of a machining operation.